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In most species, old age brings an unwelcome litany of physical changes. For humans, that might mean more wrinkles, more trips to the bathroom, and more frequently asking, “Why does that hurt?” For birds, feathers may lose their luster and even start breaking down from years of wear and tear. Recent research has also found that a bird’s song quality can decline over time. 

But until now, scientists had been unable to determine whether birds—or any animals for that matter—can detect age difference through voice. A in Behavioral Ecology suggests not only that they can, but a bird’s ability to distinguish the young from the old through song likely plays an important role when it’s time to establish territory and compete for mates. 

The study, co-authored by Matthew Zipple and Stephen Nowicki from Duke University, builds on the pair’s previous that first identified song degradation in Swamp Sparrows. While the difference in calls between the old and young sparrows is imperceptible to human ears, Zipple and Nowicki used spectrograms to determine that the older Swamp Sparrows sing less often and are more inconsistent than younger birds when singing the repeated notes that make up a Swamp Sparrow's song. This phenomenon is similar to how a human voice can start to quaver in old age.  

With this knowledge, Zipple and Nowicki set out to discover whether Swamp Sparrows can perceive these subtle changes. Donning hip waders and wearing blaze orange for safety, they conducted their study this past spring in a Pennsylvania marsh on state game land. There, using field markers, they measured how close an audience of 35 wild male Swamp Sparrows encroached on a speaker when it projected songs from a 2-year-old bird and a 10-year-old bird over five minute bursts. In humans, this age discrepancy roughly equates to a 20-year-old and a centenarian. 

Running an experiment in a marsh wasn’t without its challenges. At one point, the speaker fell into the water, sparking panic and a sudden flight to a nearby lab, where the researchers frantically set upon the waterlogged equipment with hairdryers. The scientists also took a bit of a beating over the course of the study. “Every once in a while, you're going to fall in the marsh, which did happen to me a couple of times,” Nowicki says. “And that’s the price of doing business in a marsh.”  

When the researchers analyzed the results of their experiment, they found that the Swamp Sparrows were noticeably more aggressive approaching the speaker when it played the recording of a younger bird. On average, the sparrows flew about seven feet, or two meters, closer when the speaker mimicked the 2-year-old sparrow. While that might not sound like much for a human, it’s close enough to signal an imminent attack by a rival sparrow.

“The difference of two meters for a bird could potentially be the difference between being within quick striking distance—taking a step and attacking somebody—versus having having to run at somebody in order to get there,” Zipple says. 

For both researchers, the varying approach distances were evidence that sparrows are able to differentiate between the recordings. In addition, the findings also suggest the birds felt more threatened by the younger sparrow’s song than that of the older one. This makes sense, since younger sparrows represent a far greater threat to another bird’s turf and mating success. 

Zipple and Nowicki plan to continue this line of research by examining how female Swamp Sparrows hear and react to aging sparrow voices. But that task won’t be as easy as it was with males; looking at the female response to song is trickier to measure in the wild, according to Nowicki. Instead, females will need to be brought into the lab and tested with controlled experimental methods. Despite this challenge, the scientists are eager to see how that experiment plays out. 

“It's usually the female that is much more discriminating about differences in male song,” he says. “That is an obvious next thing, to ask the females, ‘Do you care as well?’”